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Sunday, December 9 , 2018, 12:12 pm | Fair 66º


Dennis Mammana: How to Read a Star Map So the Sky Will Have No Limits

Star map Click to view larger
Read a star map after dark this week. (Dennis Mammana illustration / creators.com)

I still recall how challenging it was to find my way around the sky for the first time, so I understand what novice stargazers go through. Believe it or not, though, it really isn’t a difficult process — once we begin to recognize patterns among the stars.

And that’s where a simple star map helps.

Since Earth’s daily rotation and annual journey around the sun cause our sky to change constantly, the map here represents the sky we see during the early evening hours of June 2018.

Notice that it’s surrounded by a circle that represents the horizon — the imaginary boundary between sky and ground — and the map center marks the zenith, or overhead point. And along its edge you’ll find the cardinal directions of north, east, south and west.

Within the bounds of the star map, each black dot represents a star. The larger the dot, the brighter the star it represents. And to help organize the heavens into recognizable segments, traditional star patterns are outlined by lines. Simple enough so far.

But putting a star map to use is where many people give up. The trick is, unlike a roadmap, which is held in a normal reading position, a star map must be held overhead, with its cardinal directions aligned to the actual directions. Once we learn this, we’re halfway there.

The map can be used in two ways. First, we can go from the sky to the map. Suppose we spot a large triangle of bright stars about halfway from the eastern horizon to the zenith. A quick glance at the map shows that this grouping may be the outline of the Summer Triangle, also the home of three separate constellations.

The other way to use the map is to go from the map to the sky. Suppose we want to find the constellation Scorpius, the scorpion; the map shows it sitting low in the southeastern sky after dark in June, so that’s where we must look for it in the heavens.

Once we find one or two such star groupings — no matter which technique we choose to do it — we’ll be on our way to finding more. Now we can use those as markers in the sky to “hop” our way around to others.

The biggest surprise in store for most stargazers is that the star patterns in the sky appear much larger than they do on the map. The map is, after all, only a scale model of the real sky. Once we grasp this difference in scale, finding our way around the sky becomes relatively simple.

You can find plenty of planetarium apps for your phone or tablet that you can set for your exact location, date and time. They can be rather small to read in the dark, however, so I always prefer paper star maps and a red flashlight (to preserve night vision).

To get a star map for each month, you can check the current issues of Sky & Telescope and Astronomy magazines at your local library or bookstore. Or you can visit skymaps.com, where you can download the current month’s map for free.

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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